1. Field of the Invention
The present invention relates to high density silicon nitride sintered bodies having excellent mechanical strength, oxidation resistance and static fatigue resistance.
2. Related Art Statement
Since silicon nitride sintered bodies have superior mechanical strength, heat resistance, heat shock resistance and corrosion resistance compared metallic materials, silicon nitride sintered bodies have been examined for use in a variety of mechanical parts used in high temperatures where metallic materials usually fail. The materials to be used for the mechanical parts which are to operate at such high temperatures need to have excellent mechanical characteristics at high temperatures as well as high oxidation resistance and static fatigue resistance from the standpoint of durability and dimensional stability during use of the parts for a long time period.
In order to obtain high density silicon nitride bodies, there have been known a pressureless sintering process, a pressure sintering process etc. Although various kinds of articles can be easily and inexpensively mass-produced by the pressureless sintering process, this process has drawbacks in that an amount of a sintering aid necessary for obtaining high density is greater and the high temperature strength and oxidation resistance are insufficient. On the other hand, although higher density sintered bodies can be obtained with a lesser amount of a sintering aid by the pressure sintering process, it has a major drawback that production costs become extremely high in the case of mass production of large sized articles having complicated shapes.
In order to solve the problems mentioned above, the applicant has proposed, in Japanese Patent Laid-open Publication No. 60-54,976, silicon nitride sintered bodies having high density, high strength and excellent oxidation resistance.
However, although silicon nitride sintered bodies having high mechanical strength and oxidation resistance can be obtained by this method, silicon nitride sintered bodies having sufficiently satisfying static fatigue characteristics, (SCG characteristics) which are important when in use as mechanical parts working particularly at high temperatures, cannot be obtained.
In order to further solve the problems mentioned above, the applicant has proposed, in U.S. Pat. No. 4,801,565, that silicon nitride sintered bodies having a J phase solid solution in an intergranular phase have high temperature strength, excellent oxidation resistance and excellent static fatigue characteristics. However, even by the method disclosed in U.S. Pat. No. 4,801,565, there is a drawback in that satisfying high temperature strength and excellent static fatigue characteristics cannot be always obtained when a state of J phase solid solution present in an intergranular phase is varied.